Methods for thermal well logging

ABSTRACT

A method for thermal well logging of earth formations in cased or open boreholes to produce logs of temperature, specific heat and formation thermal conductivity is disclosed. A well logging tool employing a constant output heat source and three temperature sensors is utilized. A first temperature sensor measures the ambient borehole temperature at a given depth level. The constant output heat source then heats the formation at the investigated depth level. A second temperature sensor located relatively near the heat source then measures the temperature increase at the depth level due to the heating. This provides a measure of the formation specific heat.

States Unite Smith, Jr.

atent 1 [451 Apr. 30, 1974 [21] Appl. No.: 272,266

[52] US. Cl. 73/154 Primary Examiner.lerry W. Myracle Attorney, Agent,or FirmT. H. Whaley; C. G. Ries 5 7] ABSTRACT A method for thermal welllogging of earth formations in cased or open boreholes to produce logsof temperature, specific heat and formation thermal conductivity isdisclosed. A well logging tool employing a constant output heat sourceand three temperature sen- [51] Int. Cl E21d 47/06 Sors is utilized Afirst temperature Sensor measures 58 Field of Search 73/154 the ambientborehole temperature a give" depth level. The constant output heatsource then heats the [56] References Cited formation at theinvestigated depth level. A second UNITED STATES PATENTS temperaturesensor located relatively near the heat source then measures thetemperature increase at the E i: l

depth level due to the heating. This provides a meaowe e a 2,290,0757/1942 Schlumberger 73/154 Sure of the formanon Speclfic heat 2.342.8272/1944 Ackers 73/154 4 Claims, 2 Drawing Figures RELAT/ VE BOREHOLECONDUCT/ V/ TY TEMP.

- RELA T/VE SP HEAT 22 23 SUPPLIES RECORDER PRO CESS/NG SIGNAL CIRCUITSRECORDER I SUPPLIES 27 BORE HOLE TEMP.

RE LA T/ VE SP. HEAT RELATIVE CONDUCT/ V/ TY POWER 1 A i SIGNAL 9PROCESSING CIRCUITS & RELATIVE BOREHOLE CONDUCT/V/TY TEMP RELATIVE SPHEAT RECORDER #54 SIGNAL SUPPLIES 06555 G METHODS FOR THERMAL WELLLOGGING BACKGROUND OF THE INVENTION This invention relates to welllogging and more specifically to methods of thermal well logging todetermine the specific heat, thermal conductivity, and geothermalgradient present in boreholes.

In cased well bores there are relatively few physical measurements whichmay be made through the casing to determine physical properties offormations behind the casing and the cement column about the casing.Electrical logging is prohibited because the metallic properties of thecasing act as a shield on the electrical formation properties whichcould be measured. Acous-.

tic logging behind casing has been attempted but has generally beenunsuccessful, primarily because of the large. amount of acoustic energyabsorbed by the casing. Again as in the electrical measurement case,this masks or obscures the acoustic characteristics of the earthformations behind the casing.

Nuclear well logging techniques, while generally usable in a casedborehole, have frequently encountered difficulty in distinguishing freshwater from oil. Some nuclear welllogging techniques require that thesalinity of the formation waters be relatively high (greater than about40,000 parts per million sodium chloride) in order to'detect water anddistinguish it from oil. inthe pores of earth formations. Other nuclearlogging techniques not basedon salinity are effected by changes in theformation matrix type. Moreover, nuclear logging techniques generallyrequire complex and delicate downhole equipment. In addition to theseshortcomings, most nuclear Well logging instruments present at leastsome form of radiation hazard to personnel and must be handled with carein order to prevent the undesirable leakage of radioactive materials.

Thermal well logging has heretofore been conducted primarily in the formof simple temperature measurements taken in a borehole. It has beenknown in the past, for example, to measure the temperature at variousdepths in a borehole using a tool having either one or two highresolution thermometers in order to measure the geothermal temperaturegradient. Logs of the temperature alone or of the temperature gradienthave been produced. Anomalies such as a sudden slope change in thegeothermal temperature gradient have been used to detect leaks in casingor production tubing or to locate fluid flow behind the casing in acased well bore.These thermal measurements have also been utilized tolocate fluid entries into the well bore or to define the lowest depth ofproduction or liquid injection in a well. In addition to the above usesthese tech-' niques have also been utilized to locate the cement top ina cased well bore.

In the prior art temperature logging techniques, no source of heat hasbeen utilized in the well bore to provide a constant heat flow from awell logging tool into earth formations; In the present invention theuse of a constant output heat source enables earth formationcharacteristicsto be measured in either open or cased well bores.Properties which may be measured include the thermal conductivity ofearth formations and the specific heat of earth formations. Moreover,all of the prior art techniques which have been derived from simpletemperature measurements in the borehole may also be utilized in thetechnique of the present invention as the ambient temperaturemeasurements are readily available. The present invention provides amethod of detecting the difference between fresh water (or salt water)and hydrocarbons in cased well bores. This provides a basis fordetermining whether a given earth formation contains fresh water, saltwater, or hydrocarbons in the pore spaced therein.

Accordingly, it is an object of the present invention to provide methodsfor determining the specific heat of earth formations surrounding a wellbore.

It is still a further object of the present invention to provide a meansfor determining whether the fluid content of earth formations in a casedborehole is fresh water, salt water, or hydrocarbon.

It is a further object of the present invention to provide anon-hazardous well logging technique for distinguishing fresh waterfilled earth formations from hydrocarbon filled earth formations whichis substantially independent of formation lithology and boreholeeffects.

The above and other objects, features and advantages of the presentinvention are provided by methods for thermal borehole logging whichincludes a constant output heat source and plural thermal detectors. Awell logging tool containing a heat source and plural temperaturedetectors is moved vertically through a borehole. A first temperaturedetector detects the ambient borehole temperature at each depth level.Two spatially separated detectors then determine the effect on thetemperature of earth formations surrounding the borehole caused by theheat source contained in the tool. The I temperature measurements madeby the three detectors may then be appropriately combined to provideplots, graphs or logs indicative of the borehole temperature, and theearth formation specific heat for formations in the vicinity of theborehole. In another embodiment of the invention athermal drilling toolor heat drilling tip is utilized for oil well excavation. Pluraltemperature detectors spaced longitudinally from the thermal drillingtip monitor the borehole temperature at three different spatiallocations removed from the vicinity thermal drilling tip or heat source.This provides, While'drilling, a log of the borehole temperature,thermal conductivity, and specific heat as a function of depth when themeasurements from these temperature detectors are appropriatelycombined. 4

The present invention is described with particularity in the appendedclaims. The invention is best understood when the following detaileddescription thereof is taken in conjunction with the associated drawingin which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showingthe method of the present invention being utilized in borehole loggingof a cased well bore.

FIG. 2 is an embodiment of the present invention utilized in a thermaldrilling operation for a borehole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Two physically measurablequantities in both cased and uncased boreholes are the specific heat (C)and thermal conductivity (K) of the formation matrix and fluid. Theseparameters constitute the basis of the logging techniques of the presentinvention. In a borehole environment the rise in temperature (AT), isrelated to the radiated energy (Q) from a heat source, the heated earthformation (M) and its composite specific heat (C) as given by therelationship of equation (l):

Q= C'M'AT If a well logging tool having a constant output heat source ismoved past an earth formation in a borehole, the rise in temperaturethus would be inversely proportional to the specific heat of theborehole and formation surrounding the borehole at a particular depthlevel. In Table l the specific heats (C) for commonly encounteredborehole materials are listed.

The values given in Table l are for the range of temperatures from 20 to100 C. It will be noticed by observing the data of Table I that all ofthe formation matrix materials have approximately the same specific heat(i.e., about 0.20 Cal/gm C). Secondly, there is a strong difference(i.e., a factor of two) between the specific heat of petroleum andwater. Thirdly, specific heat is not a strong function of water salinity(typically variations on the order of percent or less).

Accordingly, if it is possible to measure the relative specific heatof'earth formations surrounding a well borehole, this information. canbe useful when combined with other logs in determining the fresh water(or salt water) zones and differentiating these zones from petroleumcontaining zones. This has long been a problem in the logging of casedboreholes due to the relatively similar response of fresh water andpetroleum to conventional neutron lifetime or thermal neutron decay timewell logging techniques.

Referring now to FIG. 1, apparatus is illustrated schematically forperforming a temperaturelog and a specific heat log of earth'formationssurrounding a well borehole. A borehole 2 penetrating earth formations 3is lined with a steel casing 4 which is cemented in place by a cementlayer 5. Borehole fluid 6 fills the borehole.

A fluid tight well logging sonde 7 is suspended via a well logging cable8 which passes over a sheave wheel 9 in the borehole 2. The loggingsonde 7 is eccentered,

well logging sonde 7. The body of the fluid tight sonde 7 is alsopreferably constructed of a thermally insulating material to preventheat passage therethrough.

The heat sensitive transducers 12, 13 and 14 may comprise thermallysensitive resistors or thermistors which are connected as one leg of abalanced bridge.

circuit if desired. This technique is well known in the art for accuratemeasurement of borehole temperature. Other known temperature sensitivetransducers may be utilized for this purpose. Such an alternativetransducer could comprise a high frequency oscillator whose frequency isdetermined by a resistive element which is temperature sensitive.Alternatively, thermocouple type heat sensitive transducers may be usedif desired.

It will be appreciated by those skilled in the art that the downholesonde 7 also contains appropriate power supply circuitry and datatransmission circuitry (not shown) for operating the heat source 11 andtemperature sensitive transducers 12, 13 and 14 and for transmittingmeasurements made thereby to the surface on conductors of the welllogging cable 8 for further processing. Any of a number of known datatransmission and power transmission systems used in well logging may beutilized for this purpose in the present invention. The electrical.power to operate the downhole sonde is supplied by a surface powersource 19. Signals from'the downhole instrumentation are supplied tosignal processing circuits 20 whose function will be subsequentlydescribed in more detail and a well log of the relative conductivity,relative'specific heat and borehole temperatures are provided from thesignal processing circuits to a recorder 21. Recorder 21 is driven by anelectrical or mechanical linkage with the sheave wheel 9 (as indicatedby the dotted line 22) as a function of borehole depth. The recordmedium 23 for plotting these logs is moved as a function of boreholedepth. The sheave wheel 9 may also be mechanically or electricallyconnected to the signal processing circuits 20 in order that depthinformation may be utilized for computing the relative formationconductivity and specific heat in the manner to be described. This isindicated by the dotted line 23 in FIG. 1.

Power from the surface power supplies 19 is supplied to the downholecircuitry and to a heat source 11 in the downhole sonde. The heat source11 may comprisea heater coil or other electrical heater apparatus whichsupplies a constant rate of heat output. Thus, as the sonde 7 is movedvertically past earth formations 3 at a constant speed a given or knownamount of heat Q is generated from the heat source 11 and is supplied tothe formations primarily by conduction and convection. t

As a given amount of heat is applied from the downhole sonde 7 to theearth formations 3 surrounding the borehole 2, the rise in temperatureinduced at each depth can enable oil or gas bearing strata to bedetermined from water bearing strata in the following manner. It will berecalled from Table I that changes in water salinity and formationmatrix are negligible on the specific heat of the earth formations inthe vicinity of a borehole relative to changes in hydrocarbon content.Borehole effects, although not negligible, should be relativelyconstant, providing there is no change in casing, borehole fluid, orannular cementation. Therefore, different temperature changes in variousdepth zones when supplied with a constant amount of heat as provided bythe sonde 7 can be attributed to differences in the hydrocarbon contentin the pore spaces of these various zones. Smaller temperature changeswill occur in water bearing zones than in those containing hydrocarbon.For example, in the idealized case of a percent porosity formationsaturated in the first case with water and in the second case withhydrocarbon and for a constant output heat source such as that in thesonde 7 the temperature change AT in the water saturated formation isgiven by:

water Q/ water Water Q/ water Where M is the mass of the heated materialand C is the specific heat of the water saturated formation. Similarly,the temperature change caused by supplying the same amount of heat Q toan oil saturated formation is given by:

m-z Q/ ou 011 Q/ on Where pWater and pOilare the densities of water andoil. It may thus be seen from equation 4 that in this idealized case thetemperature change from the ambient temperature condition will beapproximately 2.2 times greater for a pure oil bearing formation of 100percent porosity than will be the case for a pure fresh water bearingformation in, the same porosity. Of course it will be appreciated bythose skilled in the art that this idealized case is not generallyencountered in a borehole environment. However, it is representative ofthe type of measurement which may be expected using the methods of thepresent invention.

In making the measurement of the temperature change from the ambientcondition to that caused by the application of constant heatsource l 1,the temperature at each borehole depth from the upper or ambienttemperature sensor 12 of FIG. 1 must be stored in a memory device at thesurface until the heat source 11 has passed this depth point and thesecond temperature sensor or detector 13 has reached the same depthlevel. When this occurs, the ratio of the temperatures caused by theconstant output heat source 11 is detected. If the formation density atthis depth is known, as for example by a gamma-gamma density log, thespecific heat of the formation relative to the specific heat of Watermay be computed from equation 4. This computation may be performed inthe surface signal processing circuits 20 of F IG. 1. Such circuits cancomprise a small general purpose digital computer programmed to performthe computations of equation (.4).

Referring now to FIG. 2, another embodiment of the present invention isshown in conjunction with borehole drilling apparatus. In theconfiguration of FIG. 2, a borehole 42 isbeing drilled in earthformations 43 by a thermal boring device 44 comprising a constant outputheat source. Such av device is shown, for example, in the article inNUCLEAR NEWS, Volume 15, No. 2, February 1972, p. 47. A very high outputheat drilling tip isused, to virtually melt the earth formations therebyproducing a borehole. The drilling rig is suspended from a derrick 45via a wire line 46 which also serves to conduct electrical power fromthe surface power supply 47 to the thermal boring device 44. Cable 46also serves to conduct signals from temperature sensors 48, 49 and 50 tothe signal processing circuits 51 which 'are situated at the surface.Cable 46 passes over a sheave wheel 53 which may (in a manner similar tothat of sheave wheel 9 of FIG. 1) be mechanically or electricallycoupled to a recorder 54 for recording the output of signal processingcircuits 51 as a function of borehole depth.

In the embodiment shown in FIG. 2, the ambient temperature detector 50is situated a sufficient distance from the thermal drilling tip 44 toallow the formations to return to their ambient temperature afterpassage of the heat drilling tip 44 (possibly recorded in another passthrough the borehole at a subsequent time). Temperature sensors 48 and49 are analogous to temperature sensors 13 and 14 of FIG. 1 and aresituated close enough to the heat drilling tip 44 to permit the specificheat measurements to be made in the manner previously described withrespect to FIG. 1. Signal processing circuits 51 may again comprisedigital computer means which are programmed to perform the calculationsfor the relative specific heat based on the temperature measurements ofthe three sensors 48, 49 and 50 and previously described with respect tothe corresponding temperature detectors l2, l3 and 14 of P10. 1. Thus,measurements indicative of the borehole temperature and relativespecific heat, may during the drilling process when utilizing the heatdrilling tip 44 may be made. The log of these quantities recorded by therecorder 54 as a function of borehole depth.

The foregoing descriptions may make other alternative embodiments of theinvention'apparent to those skilled in the art. It is the aim in theappended claims,- therefore, to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

1 claim:

1. A method for thermal logging of a well borehole to provide a measureof the specific heat of earth formations in the vicinity of the boreholecomprising the steps of:

measuring the ambient temperature of earth formations in the vicinity ofthe borehole at a selected depth level; heating the earth formations atsaid selected depth level by imparting a predetermined quantity of heatthereto; 1 measuring the temperature of the earth formations at saidselected depth level substantially simultaneously with said heating stepto determine the temperature increase therein due to said heating step;and combining according to a predetermined relationship said first andsecond temperature measurements at said depth level to produce anindication of formation specific heat at said depth level.

2. The method of claim 1 wherein said heating step is performed byapplying heat from a relatively constant output heat source to saidformations for a predetermined interval of time.

3. The method of claim 1 wherein the steps are performed at a pluralityof levels in a well borehole by moving a well logging sonde having anambient temperature sensor, a heat source and a heated formationtemperature sensor past a given depth level, making the ambienttemperature measurement at a given level and storing said measurementuntil said heating and heated ship said measurements to produce anindication of formation specific heat at said plurality of depth levels.

4. The method of claim 3 and further including the step of recordingsaid specific heat indications as a formation temperature measurementsare made and function of borehole depth.

then combining according to a predetermined relation-

1. A method for thermal logging of a well borehole to provide a measureof the specific heat of earth formations in the vicinity of the boreholecomprising the steps of: measuring the ambient temperature of earthformations in the vicinity of the borehole at a selected depth level;heating the earth formations at said selected depth level by imparting apredetermined quantity of heat thereto; measuring the temperature of theearth formations at said selected depth level substantiallysimultaneously with said heating step to determine the temperatureincrease therein due to said heating step; and combining according to apredetermined relationship said first and second temperaturemeasurements at said depth level to produce an indication of formationspecific heat at said depth level.
 2. The method of claim 1 wherein saidheating step is performed by applying heat from a relatively constantoutput heat source to said formations for a predetermined interval oftime.
 3. The method of claim 1 wherein the steps are performed at aplurality of levels in a well borehole by moving a well logging sondehaving an ambient temperature sensor, a heat source and a heatedformation temperature sensor past a given depth level, making theambient temperature measurement at a given level and storing saidmeasurement until said heating and heated formation temperaturemeasurements are made and then combining according to a predeterminedrelationship said measurements to produce an indication of formationspecific heat at said plurality of depth levels.
 4. The method of claim3 and further including the step of recording said specific heatindications as a function of borehole depth.